A customer leaving the post office at 1750 Lundy Ave. saw a man wearing a gas mask and what looked like a bulletproof vest, said Gus Ruiz, a spokesman for the postal service's Bay Valley district.
San Jose police Sgt. Jason Dwyer said that a passerby reported seeing a suspicious person run up to the box wearing a mask, drop something inside it and run away at 12:16 p.m.
Customers and workers were evacuated from that part of the building, Ruiz said.
A robot and an officer wearing a protective blast suit checked the mail collection boxes and it was determined that the package was harmless, Dwyer said.
Police were able to single out the suspicious package and the rest of the mail was unharmed, Ruiz said.
Gus Ruiz said he has been with the U.S. Postal Service's San Jose branch since 1984, and says that no other bomb scares have occurred under his watch.